1. Field of the Invention
The present invention relates to a pressure-contact type semiconductor device and more particularly to an emitter voltage sensing electrode in MOS gate driving switching devices used in, for instance, IGBT (Insulated Gate Bipolar Transistor), thyristor, etc.
2. Description of the Related Art
FIG. 1 shows a schematic pattern layout on conventional pressure-contact type IGBT chips. The cross-sectional structure along the line B-B in FIG. 1 is schematically shown in FIG. 2, and an equivalent circuit of said pressure-contact type IGBT is shown in FIG. 3.
In FIG. 1 and FIG. 2, 60 shows an IGBT chip. On the top of this IGBT chip 60, there is formed an emitter electrode array 62 and a gate electrode 63 separately insulated from each other. 64 is a chip frame made of electrically insulate material to hold the outer edge of said IGBT chip 60; 65 is a first Mo (Molybdenum) plate so arranged that its one side is brought in contact with said emitter electrode array 62 of said IGBT chip 60; and 66 is a Cu emitter electrode plate so arranged that the other side is brought in compression contact with said first Mo plate 65.
67 is a second Mo plate so arranged that its one side is brought in contact with a collector electrode formed on the back of said IGBT chip 60, and 68 is a collector electrode plate so arranged that the other side is brought in compression contact with said second Mo plate 67.
69 is a gate electrode pull-out lead wire of which one end is brought in compression contact with a gate electrode 63 of said IGBT chip 60, the other end is taken out to the outside and its middle portion is covered by, for instance, an insulated tube 70 and is passing through a space enclosed by said chip frame 64 and emitter compression contact plates (the first Mo plate 65, the Cu emitter electrode plate 66).
Further, E is an emitter terminal connected to said Cu emitter electrode plate 66; C is a collector terminal connected to said collector electrode plate 68; and G is a gate terminal connected to said gate electrode lead wire 69.
In case of the pressure-contact type IGBT in said structure, an emitter compression contact plate composed of a single Mo plate 65 and a single Cu emitter electrode plate 66 is brought in compression contact with said emitter electrode array 62 on a single chip 60 and a collector compression contact plate composed of a single Mo plate 67 and a single collector electrode plate 68 is brought in compression contact a collector electrode on the back of the chip 60.
Therefore, like the equivalent circuit shown in FIG. 3, there exist inductance component L1 of the Cu emitter electrode plate 66 and inductance component L2 of the first Mo plate 65 in series between the emitter terminal E and the emitter electrode array 62 of the IGBT chip 60, and there exist inductance component L3 of the collector electrode plate 68 and inductance component L4 of the second Mo plate 67 in series between the collector terminal C and the collector electrode of the IGBT chip 10.
In a device using said pressure-contact type IGBT, an emitter voltage sensing terminal is needed to sense (monitor) voltage of an emitter electrode of the IGBT in order to detect overcurrent when applied to the IGBT. In a conventional compression contact structure, an emitter terminal ES is connected to the Cu emitter electrode plate 66.
However, as there exist inductance component L1 of the Cu emitter electrode plate and inductance component L2 of the first Mo plate in series between said emitter voltage sensing terminal ES and said emitter electrode array 62 as shown in FIG. 3, voltage is induced on said inductances L1 and L2 when emitter current changes.
In this case, if a time change of emitter current i like turn ON/OFF or current oscillation when the IGBT is in operation is expressed by di/dt, induced voltage V between both ends of said inductance components L1, L2 is expressed by the following expression: EQU V=-(L1+L2)di/dt
Here, if di/dt is large, a large induced noise is generated between the emitter terminal and the emitter electrode 62; an error is produced between a measured voltage value of the emitter terminal and a true emitter voltage value of the emitter electrode 62; and it becomes impossible to detect emitter voltage accurately. In this connection, in case of a device which operates at a high speed like an IGBT, di/dt is large and if it is operated at, for instance, 10000 A/us, induced noise as large as about 20 V is generated even when an inductance is as small as (L1+L2)=2nH.
As described above, in case of a conventional pressure-contact type semiconductor device in such a structure that an emitter compression contact plate is brought in compression contact with the emitter electrode array on a single semiconductor chip, there was such a problem that a large induced noise is generated if a time change of emitter current is large and it will become impossible to detect emitter voltage accurately as there exist inductance components of the emitter compression contact plate between the emitter terminal and the emitter electrode array.